This application is based upon and claims priority of Japanese Patent Application Nos. 2001-262160, filed on Aug. 30, 2001, and 2002-180425, filed on Jun. 20, 2002, the contents being incorporated herein by reference.
1. Field of the Invention
The present invention relates to a thin-film type semiconductor device having an operating semiconductor layer and a method of production thereof. In particular, it is favorable to apply to a thin-film transistor in which a source/drain is formed on the operating semiconductor layer and a gate electrode is formed on a channel area.
2. Description of the Related Art
Since a thin-film transistor (TFT) is formed into a very thin fine operating semiconductor layer, mounting on a large screen liquid crystal display panel and the like is expected considering recent demand for a big area.
For a TFT operating semiconductor layer, use of a polycrystalline silicon layer has been studied because it has high carrier mobility and is thermally stable compared to an amorphous silicon layer (a-Si layer). At present, the methods shown below are used for forming method of the operating semiconductor layer using the polycrystalline silicon layer.
(1) A method of forming the polycrystalline silicon layer by heating the a-Si layer between about 600xc2x0 C. and about 1100xc2x0 C. to crystallize has been adopted. This method plans to form a crystal nucleus at an early stage of the heating process and to crystallize by growing this crystal nucleus.
(2) The a-Si layer is melted by adding laser energy to form the polycrystalline silicon layer by crystallizing it when it is cooled.
(3) The polycrystalline silicon layer is formed directly by a chemical vapor growth method or a physical evaporation method at a temperature of 600xc2x0 C. or more.
Here, using a method of forming a thin-film semiconductor layer on a glass substrate as an example, disadvantages of the conventional art are discussed. Since glass is used for a substrate material, the temperature of the substrate is limited to 600xc2x0 C. or less.
The crystal growth method described in (1) requires a heat processing temperature of 600xc2x0 C., which corresponds to heat processing at a high temperature for the glass and causes deformation of the glass. In addition, stacking faults and twin crystals are contained in the grown crystal in large quantity, which makes it impossible to expect formation of a polycrystalline silicon layer with excellent crystallinity.
With the crystal growth method described in (3), a columnar crystal is formed, which is insufficient in crystallinity because of its small crystal grain diameter, and a crystal showing a high mobility can not be formed.
With the method using laser anneal described in (2), a laser which can be used without raising the temperature of the substrate is limited to an excimer laser. When the excimer laser is used, a polycrystalline silicon layer of high quality can be obtained, because the crystal grows through a molten phase. However, it has a disadvantage in that the energy range for obtaining a polycrystalline silicon layer of such a high quality is extremely narrow. Furthermore, when the excimer laser is used, though only the thin-film silicon region on the surface layer is melted getting a high temperature, the temperature of the glass itself is low. Accordingly, the cooling rate of the silicon molten is high.
Therefore, growth from the molten occurs under a super cooling condition and many crystal nuclei are formed which makes the crystal grain diameter small. Generally, the range of the crystal grain diameter is about 300 nm to about 600 nm. When a thin-film polycrystalline silicon layer is formed using the excimer laser, which assures the best crystallinity, the mobility of the thin-film transistor is about 200 cm2/Vs, which is far smaller compared to the mobility of a single crystal silicon of 600 cm2/Vs. This is because the crystal grain diameter is small and the crystal grain boundary portion works as a scatterer with a strong carrier.
As above, there have conventionally been a serious disadvantage in that even the operating semiconductor layer is composed of the polycrystalline silicon layer, the lowering of the mobility due to the crystal grain boundary can not be restrained and the production of a high quality operating semiconductor layer can not be guaranteed.
An object of the present invention is to provide a thin-film type semiconductor device which realizes extremely high mobility by forming an operating semiconductor layer from a thin-film semiconductor layer having a negligibly small effect of a crystal grain boundary, and to provide a method for production of the semiconductor device which enables the semiconductor device to be produced easily and securely.
The present invention provides the following embodiments to solve the above-described disadvantages.
The present invention relates to a thin-film type semiconductor device provided with an operating semiconductor layer on a substrate and a method for production thereof.
In the semiconductor device of the present invention, the operating semiconductor layer is shaped so that a wide region and a narrow region are connected with each other. The wide region is in a flow pattern state with a large crystal grain, and a direction of a crystal grain boundary in the above-described flow pattern is formed not to be parallel to a longitudinal direction of the narrow region, while the narrow region is substantially in a single crystalline state.
The method for producing the semiconductor device of the present invention comprises the steps of: forming a semiconductor layer which is to be the operating semiconductor layer above the substrate; processing the semiconductor layer to be shaped so that it has a wide region and a narrow region and the narrow region is connected to the wide region in a manner that the narrow region is positioned to be asymmetric with respect to the wide region, and crystallizing the semiconductor layer by irradiating energy beam to the semiconductor layer along the longitudinal direction of the narrow region from the wide region toward the narrow region.
Another aspect of the method for producing the semiconductor device of the present invention comprises the steps of: forming a semiconductor layer to be the operating semiconductor layer above the substrate; processing the semiconductor layer to be shaped so that it has a wide region and a narrow region; and crystallizing the semiconductor layer by irradiating energy beam to the semiconductor layer in such a manner that a scanning plane of a beam spot is inclined from a perpendicular position to a longitudinal direction of the semiconductor layer.
From the above consideration, if an outbreak of the crystal grain boundary bringing about lowering of the mobility can be restrained, the mobility is improved and performance of the semiconductor element can be improved. For this purpose, it is satisfactory to structure the operating semiconductor layer from a crystal grain having a large crystal grain diameter and the ultimate figure of the operating semiconductor layer is a complete single crystal semiconductor.
In the semiconductor device according to the present invention, the operating semiconductor layer is structured such that the wide region is in a crystalline state with a flow pattern having a large crystal grain and the narrow region is substantially in a single crystalline state. Since there exists actually no crystal grain boundary due to the flow pattern in the narrow region, therefore, when the narrow region is used as a channel, a semiconductor device having a high mobility can be obtained without fail.
In a method of production of the semiconductor device according to the present invention, energy beam is irradiated to the semiconductor layer, which is patterned to be shaped so that the narrow region is connected to the wide region in a manner that the narrow region is positioned to be asymmetric with respect to the wide region, along the longitudinal direction of the narrow region from the wide region toward the narrow region. At this time, the wide region is solidified along the direction of the irradiation, and a flow pattern including crystal grains having a large diameter controlled in growth direction is formed in the wide region. The crystal grain boundary is formed toward the narrow region in the flow pattern. Though the shape of the crystal grain boundary formed with whole of respective flow patterns is symmetric with respect to the direction of the irradiation, since the narrow region is formed to be asymmetric with respect to the wide region, the crystal grain boundary collides against a wall in the narrow region near the boundary between the narrow region and the wide region and disappears with high possibility, which restrains formation of the crystal grain boundary in the narrow region. Owing to this effect, an operating semiconductor layer in which the wide region is in a crystalline state having a flow pattern with a large crystal grain and the narrow region is substantially in a single crystalline state is to be formed.
In another aspect of the method for producing the semiconductor device of the present invention, energy beam is irradiated to the semiconductor layer, which is patterned to be shaped so that the narrow region and the wide region are connected to each other in a manner that the narrow region is positioned to be asymmetric with respect to the wide region, in such a manner that a scanning plane of a beam spot is inclined from a perpendicular position to a longitudinal direction of the semiconductor layer. At this time, the wide region is solidified along the direction of the irradiation, and a flow pattern including crystal grains having a large diameter controlled in growth direction is formed in the wide region. The crystal grain boundary is formed toward the narrow region in the flow pattern. Though the shape of the crystal grain boundary formed with whole of respective flow patterns is symmetric with respect to the direction of the irradiation, since an inclination angle is given to the scanning plane of the beam, the crystal grain boundary securely collides against a wall in the narrow region near the boundary between the narrow region and the wide region and disappears, which restrains formation of the crystal grain boundary in the narrow region. Owing to this effect, an operating semiconductor layer in which the wide region is in a crystalline state having a flow pattern with a large crystal grain and the narrow region is substantially in a single crystalline state is to be formed.
The methods of production of the semiconductor device according to the present invention further comprise the steps of: forming a heat-retaining layer so as to cover only a side portion of the narrow region selectively; and irradiating energy beam in this state. Then, the heat-retaining layer serves as a heat reservoir having a large heat capacity to make cooling rate of a molten small and to control the heat distribution of the semiconductor layer so that the position of nucleus formation and direction of crystal growth are controlled. In this case, crystallization proceeds by progress of the temperature lowering from a center portion of the narrow region. However, since the side portion of the narrow region is selectively covered with the heat-retaining layer, it is the most difficult to lower the temperature of this side portion and effective crystallization can be realized. Therefore, the crystalline state having a large crystal grain diameter can be realized with high reliability.